I am no expert, so I will quote from Mrs. Fonstadt's Atlas of Middle-earth:
Quote:
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East of the Misty Mountains the moderating marine influence was lost. As Aragorn said during the boat trip south: "[We] are far from the sea. Here the world is cold until the sudden spring." Even in Rohan there was snow cover from November to March in the Long Winter, although it was normally mild, as Aragorn, Legolas, and Gimli found it in late February. Mountains often produce steppe, or even desert, climates on their leeward sides; yet this was not the case of either the Misty or White Mountains. The grasslands of Rohan could have resulted from this rain-shadow effect, but east of the Misty Mountains lay widespread forests — perhaps due to less evaporation in the cooler air of the north.
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Quote:
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Mid-latitude climates are the battlefield between cold polar and warm tropical air masses, producing a belt of cyclones that move from west to east across the lands. With these go the familiar fronts: Cold Fronts, with associated thunderstorms and driving rain; Warm Fronts, with gentle warm summer rains; and Occluded Fronts, with all-day winter drizzles.
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On the climate map Rhovanion fits the second quote, being partly in the cold winter, mild summer and partly in the cold winter, warm summer area. The line dividing them goes from the souther tip of the Sea of Rhun to the High Pass.
I can well relate to this climate area, since I come from Romania. This is the typical South-Eastern European climate - with cold winters, with cold winds blowing from Russia, in Rhovanion's case from the north and east.
In summer however it can get quite warm.